Adolescence has only recently been recognized as a protracted period of extensive brain remodeling. This critical period of development is associated with the emergence of sex differences in susceptibility to and manifestation of several mental illnesses, including eating, mood, and conduct disorders, and schizophrenia. Thus, the etiology of these illnesses is likely to be impacted by how pubertal hormones influence the remodeling of sexually differentiated behavioral circuits during adolescence. The longstanding view has been that sexual differentiation of the brain occurs during late embryonic or early postnatal brain development, and that these sex differences are passively maintained throughout adolescence and into adulthood. Recent findings from the PIs' laboratories overturn this view by providing evidence in rats for active maintenance of sexual dimorphisms during puberty via active and hormonally modulated cell addition in sexually differentiated cell groups during puberty. This finding represents a fundamental shift in the understanding of how and when sexual dimorphisms in the brain are established and maintained in the mammalian brain. This newly discovered developmental process may be an active mechanism for either maintaining structural and functional sexual dimorphisms in the face of remodeling of the adolescent brain or for creating new sex differences that emerge during adolescent development. Using timed injections of bromo-deoxyuridine (BrdU) coupled with immunohistochemistry for markers of neurons and glial cells, as well as functional assays, the mechanisms underlying this addition of new cells to the adolescent brain will be determined. The questions to be addressed are: 1) Do gonadal hormones modulate the pubertal addition of cells to the adolescent brain in cell groups that are either male-biased or female-biased by increasing cell proliferation, survival, or both? 2) Is addition of new cells a pubertal or a life-long mechanism for maintenance of structural sexual dimorphisms? 3) What are the fates and functional outcomes of cells that are added to the adolescent brain during puberty? These studies will generate new knowledge and potentially new therapeutic targets to explain and appropriately treat sex-biased mental illnesses that are associated with puberty and adolescence.